This invention relates to a method and an apparatus for detecting coatings and has particular application, for example, to plain margin detectors for the detection of an acceptable plain margin that is to be joined in can production.
A discussion of a typical manufacturing process for three-part cans, and a discussion of at least some of the problems encountered in welding a blank into a tube associated with a failure to obtain a good plain margin of unlacquered sheet, is given later with reference to FIGS. 1 to 4 of the accompanying drawings. The reader is directed to read this text now in order to put the invention into context.
It is an object of the invention to provide an apparatus and a method for detecting a coating whether that coating is wet or dry.
According to one aspect of the present invention there is provided an apparatus for detecting a coating on a substantially planar object which is moving in a predetermined direction relative to the apparatus, the object having a coating on at least a part of the planar surface thereof, which coating is at least partially transparent to visible radiation, the apparatus comprising:
an emitter adapted to emit a beam of substantially non-visible electromagnetic radiation having a predetermined wavelength range towards the object;
scanning means adapted to scan the beam of electromagnetic radiation in a direction that has at least a component transverse to the predetermined direction;
a sensor for detecting radiation in the predetermined wavelength range which is reflected in a specular manner by the object; and
means for determining the presence and/or absence of a coating on the object on the basis of the magnitude of specularly reflected radiation.
According to another aspect of the present invention there is provided a method of detecting a coating on a planar object which is moving in a predetermined direction, the object having a coating on at least a part of the planar surface thereof, which coating is at least partially transparent to visible radiation, the method comprising the steps of:
emitting a beam of substantially non-visible electromagnetic radiation having a predetermined wavelength range towards the object;
scanning the beam of electromagnetic radiation in a direction that has at least a component transverse to the predetermined direction;
sensing radiation in the predetermined wavelength range which is reflected in a specular manner by the object; and
determining the presence and/or absence of a coating on the object on the basis of the magnitude of specularly reflected radiation.
When embodied in a plain margin detector, this enables us to check sheets of lacquered (coated) sheet material (e.g. steel) whilst the lacquer is wet or dry, preferably when the lacquer is wet immediately after lacquering.
Thus, in practice the radiation is differentially absorbed/reflected between an uncoated region and a coated region, and the sensor is adapted in use to detect that radiation.
Preferably the sensor is an optical sensor.
Preferably the radiation is absorbed by the coating more than it is by the substrate material of the object.
Preferably the radiation is emitted at a wavelength that is at, or near enough to, the absorption wavelength of a component of the lacquer, any solvent used to apply the lacquer, or the substrate material, so that it is in use absorbed. For example, the radiation wavelength may be in the near infrared or in the ultraviolet region of the spectrum.
When in the near infrared region, the radiation wavelength may be substantially at the absorption wavelength of the Cxe2x80x94H bond (on at least one of its absorption wavelengths). Most lacquers, e.g. phenolic lacquers, epoxy lacquers and vinyl lacquers, have Cxe2x80x94H bonds. Steel, of course, does not (nor does any metal).
Looking for a Cxe2x80x94H bond is a very good way of detecting whether or not there is a coating on an object, for example of detecting a plain margin and checking that the plain margin has no lacquer thereon. Conversely, it is also a good way of detecting uncoated regions in a coated object. The precise absorption wavelength of a Cxe2x80x94H bond depends upon its environmentxe2x80x94what structural groups are next to the Cxe2x80x94H bond in its molecule and even to some extent what other chemicals (e.g. solvents) are present. I have found that 3.3 xcexcm is a suitable wavelength for all, or at least a significant number of, common coating lacquers used in the production of cans. When employing near infrared radiation I can therefore use radiation of 3.3 xcexcm, plus or minus 0.1, or 0.2, or 0.3 xcexcm. Nevertheless, I have also found that 2.3 xcexcm (plus or minus 0.1, or 0.2, or 0.3 xcexcm) also works well in the near infrared region of the electromagnetic spectrum.
Alternatively, many lacquers are water-based and water absorbs strongly at 1.96 xcexcm and this represents another very good way of detecting whether or not there is a coating on an object, for example of detecting a plain margin and checking that the plain margin has no lacquer thereon. Alternatively, it is a good way of detecting uncoated regions in a coated object. Thus, I can also employ near infrared radiation of 1.96 xcexcm, plus or minus 0.1, or 0.2, or 0.3 xcexcm.
Alternatively, I have found that most coating materials absorb electromagnetic radiation in the region of 200 to 360 nm, while metals do not. Thus, looking for absorption or reflection in the region of 200 to 360 nm, preferably about 254 nm because monochromatic light eliminates aberrations, is another very good way of detecting whether or not there is a coating on an object, once again for example, of detecting a plain margin and checking that the plain margin has no coating thereon. Conversely, it is also a good way of detecting uncoated regions in a coated object. We have found that the range of 200 to 360 nm is a suitable wavelength for all, or a significant number of, common coating lacquers used in the production of cans. Depending on the wavelength of radiation required, the emitter could comprise a halogen (for infrared radiation) or a low pressure mercury (for ultraviolet radiation) light source.
Alternatively, and in particular for detecting infrared radiation, I may use a solid state emitter such as a laser or a light-emitting diode (LED). A laser or an LED can have a small emitting surface area (e.g. about 1 mm2), which is easier to focus to a small point than is light from a large surface area. I prefer to chop the emitted and/or detected signal to amplify a narrow frequency band. This reduces background noise. An LED can be chopped electronically rather than mechanically. I prefer to chop electronically. The chopping frequency is preferably more than 50 kHz, and most preferably about 100 kHz or more. It is possible to chop this fast mechanically, but it is expensive to do so.
It is cheaper and more reliable to switch LEDs on and off electronically in order to chop the emitted signal. LEDs are also quieter than a mechanical chopper. Thus the sensor preferably has emitted signal chopping means, most preferably electronic chopping of an LED.
The scanning means may comprise a mechanical scanner, or an electronic scanner. An electronic scanner may comprise, for example an emitter and/or a sensor in the form of a linear array of emitters and/or sensors. A linear array of sensors may comprise, for example, a linear CCD device.
Preferably the scanning means is adapted to scan a beam of radiation back and forth (reciprocally), for example along a line. Preferably in use the object, such as a sheet of uncut blanks, and the detector experience relative movement during a scanning operation. Preferably the object is moved relative to the scanning means, but alternatively the scanning means may move relative to the object.
The transverse component of movement is preferably substantially perpendicular to the predetermined direction of movement of the object.
The scanning means preferably scans back and forth along a line at a frequency of at least 1 kHz, preferably at least 5 kHz, and most preferably at 10 kHz or above.
Preferably the scanning means and the chopping means are adapted to pulse the radiation onto object at intervals of about 3 mm or less, or about 2 mm or less, or about 1 mm or less, or about 0.5 mm or less, or about 0.3 mm or less.
Preferably the detector has radiation gathering means (e.g. a mirror), and focussing means (e.g. a mirror or a lens) to focus the specularly reflected radiation onto a radiation-sensitive transducer.
The scanner may scan electronically. It may have a plurality of emitters which emit radiation which in use falls on different regions of the object. The emitters may be operated sequentially.
Particularly, but not exclusively, for ultraviolet radiation, the radiation may be emitted in a narrow divergent beam which is reflected by one of the coated or uncoated surface (usually the uncoated surface) and absorbed by the other. The diverging beam is focussed onto a detector by a mirror or a lens. The detector may be an array of detectors which may be scanned or may be a single detector.
The means for determining the presence and/or absence of a coating on the basis of any reflected radiation may comprise signal processing means adapted to process the signals detected by the sensor. The processing means may produce a signal indicative of a fault if it concludes there is a fault in the coating on the object. The signal processing means has reference data with which it compares a received signal sequence from the sensor to determine if there is a fault in the coating or not. The reference data is updated automatically by the signal processing means.
The apparatus may be incorporated into a can producing line. Preferably the line has line control means which controls the operation of the line. The detector preferably provides signals to the line control means which uses the signals to control the operation of a coating station of the line. If a fault signal is received from the detector (which may itself have an internal protocol for avoiding false fault signals being sent to the line control means) the line control means may be arranged to stop the coating station and/or cause an alarm to be activated.